1. Field of the Invention
The present invention relates generally to distributed amplifier monolithic microwave integrated circuits (MMIC""s), and specifically to group delay equalizer circuits for distributed amplifier MMIC""s.
2. Description of Related Art
As is well-known in the art, electromagnetic waves of different frequencies experience different time delays when transmitted through a transmission system. For example, when a broadband microwave signal is transmitted through a waveguide, the lower frequency components are delayed to a different extent than the higher frequency components. If uncorrected, this difference in time delay may give rise to appreciable distortion of the signal.
The time delay present in a circuit is known as the group delay of the circuit. Group delay is a measure of the rate of change (derivative) of amplifier phase versus frequency. Group delay remains constant over a frequency range if the insertion phase (phase shift) changes linearly with frequency. For example, time delay (xcfx84) and phase shift ("PHgr") at a given frequency (ƒ) are related by the following equation:
xcfx84="PHgr"/(2xcfx80ƒ), 
where ƒ is the fundamental frequency of a waveform, and "PHgr" is phase in radians. If the phase shift increases linearly with frequency, the time delay is constant and identical at all frequency components of a complex waveform, and thus the group delay is constant. However, if the phase shift does not increase linearly with frequency, the time delay varies with frequency, and thus the group delay is not constant over all frequencies.
Group delay variation versus frequency is an important performance parameter in next-generation high speed, high data rate (e.g., data rates up to at least 40 Gbit/s) optical fiber data links and electro-optics hardware. Optical systems have frequently utilized distributed amplifiers or feedback amplifiers to amplify a digital signal for transmission over long distances. Although such previous optical systems have experienced problems resulting from group delay variation within the amplifier itself, with higher data rates (40 Gbit/s and higher) that produce short data pulses, any group delay variation within the amplifier itself may result in distortion of the data pulse. Since a short data pulse representing digital data (0 or 1) has a broad power spectrum in the frequency domain, covering a wide bandwidth, in order to transmit a short pulse over long distances with low distortion in pulse shape and pulse width, the frequencyxe2x80x94power spectrum of the pulse must be able to be amplified over a wide frequency range with a uniform, constant delay at any given frequency.
A typical approach to designing an amplifier (e.g., a distributed amplifier) to achieve near constant group delay versus frequency involves monotonically rolling off the gain of the amplifier from low to high frequency, following various filter or polynomial equation characteristics. For example, such amplifiers may have 15 dB gain at low frequencies, decreasing to 12 dB gain at 20 GHz, and to 8 or 9 dB gain at 40 GHz. Although this enables low distortion pulse amplification, the gain rolloff severely restricts the application of the amplifier to other applications, for instance, wideband test equipment, or radios, where constant gain versus frequency amplification is preferred.
Another approach to providing near constant group delay versus frequency for a distributed amplifier involves connecting the distributed amplifier to external group delay equalizer circuitry. The external group delay equalizer circuitry flattens the overall group delay characteristic of the cascade of the amplifier and the group delay equalizer. However, the external group delay equalizer, typically fabricated as a thin-film network, may be larger than the distributed amplifier monolithic microwave integrated circuit, which adds loss to the overall circuit, thereby reducing the overall amplifier gain. In addition, the increased area of the overall circuit due to the external group delay equalizer is also a disadvantage for densely packed transceivers.
A wideband distributed amplifier monolithic microwave integrated circuit (MMIC) is disclosed that contains an integrated group delay equalizer circuit to compensate for the group delay variation of the distributed amplifier circuitry. The MMIC amplifier is capable of achieving a predicted constant group delay with little variation over a broad frequency range. In addition, the group delay equalizer circuit enables the distributed amplifier to achieve flat (constant) gain over a wide bandwidth, while maintaining constant group delay. Advantageously, the group delay equalizer circuitry requires only a small portion of the total MMIC area.